Nuclear reactors may be divided into two main types, fast and thermal.

A fast reactor is a reactor in which the fission chain reaction is sustained with high-energy or high-speed (ie fast) neutrons. It is capable of converting unused fertile uranium-238 into fissile plutonium, a process known as breeding on fast breeder reactors

A thermal reactor is a reactor in which the fission chain reaction is sustained with moderated (slow or "thermal") neutrons. There are four main types in use: light water reactors, which account for over 90% of those in operation; CANDU pressurized heavy water reactors; gas-cooled reactors (GCR, and ACR); and the RBMK light water graphite reactor.